Rheumatoid arthritis (RA) is a chronic inflammatory disease, in which the lesion occurs mainly in the synovial membrane tissue of the joint, and the prevalence rate of the disease is about 1% of the population. In rheumatoid arthritis, synovitis is found in the first stage, then cartilage or bone is gradually invaded, and the joint is destroyed and deformed in the advanced stage. Furthermore, the consequence of the symptom includes various examples such as an example in which arthritis undergoes remission and reoccurrence, repeatedly, then is completely cured, and an example in which arthritis rapidly progresses.
Diagnosis of rheumatoid arthritis is carried out mainly based on symptoms. Recently, however, attention has been paid to a diagnostic method using, as a marker, an autoantibody contained in the serum of a patient. As such an autoantibody, a rheumatoid factor (an autoantibody with respect to the deformed IgG), an anti-cyclic citrullinated peptide antibody (an anti-CCP antibody), and the like, are known (see Non-Patent Literature 1).
However, in previous reports, the sensitivity of the rheumatoid factor is 75 to 80%, the specificity thereof is 50 to 70%, and the sensitivity of the anti-CCP antibody is 50 to 75%, and the specificity is 85 to 95%, which are not necessarily satisfactory (see Non-Patent Literatures 2 and 3).    Non-Patent Literature 1: Martinus A. M. et al., Arthritis Res. Ther., 4: 87-93, 2002    Non-Patent Literature 2: Avouac J. et al., Ann. Rheum. Dis. 65: 845-851, 2006    Non-Patent Literature 3: van Venrooij W J. et al. Ann. N.Y. Acad. Sci. 1143: 268-285, 2008